The invention relates to multilayer thermoplastic film structures comprising at least one low melting point outer layer, and methods of manufacturing multilayer thermoplastic film structures comprising at least one low melting point outer layer.
Thermoplastic film structures are used in a wide variety of applications. Of the many different types of thermoplastic film structures, so-called oriented polypropylene (OPP) film structures, including biaxially oriented polypropylene (BOPP) film structures, which comprise at least one polypropylene-containing layer, are a very popular choice. OPP film structures possess excellent optical and mechanical characteristics, in addition to excellent barrier characteristics, e.g. moisture-barrier characteristics. OPP film structures, however, are not ideal in every respect.
For example, packaging applications, lamination applications, and metal adhesion applications are just some examples of the wide variety of applications in which thermoplastic film structures may be employed. For many packaging applications, the film structure must exhibit good sealing characteristics. For lamination applications, the film structure must achieve strong lamination bonds with the substrate to which it is being laminated. The metal layer of a metallized film structure must strongly adhere to the metallized layer of the film structure.
The extension of unmodified OPP film structures into packaging, lamination, and metal adhesion applications has been somewhat hampered by the difficulties associated with providing an OPP film structure that possesses: adequate heat-seal characteristics over a wide temperature range for packaging applications; adequate lamination bond strengths for lamination applications; or adequate metal adhesion for metallized film structures.
One solution has been to apply a coating having excellent heat-sealing, laminating, and/or metallizing characteristics onto an outer surface of an OPP film structure. An example of such a coating is a coating comprising a low melting point polymer.
According to one approach, a coating comprising a low melting point polymer is applied onto an outer surface of a polypropylene-containing film structure via an off-line coating process. For example, if the goal is to provide a biaxially oriented film structure having a coating on an outer surface thereof, an off-line coating process would apply the coating to the film structure after the film structure has been formed and oriented in both the machine and transverse directions. Off-line coating, however, can be extremely costly, requiring expensive equipment, such as drying ovens, solvent recovery systems, and the like.
According to a different approach, a coating comprising a low melting point polymer may be applied onto an outer surface of a polypropylene-containing film structure via an in-line extrusion coating process. In an in-line extrusion coating process, which may be used in conjunction with a sequential tenter frame orienter, a coating comprising a low melting point polymer is applied onto an outer surface of a polypropylene-containing film structure after the film structure has been formed and then oriented in the machine-direction, but before the film structure has been oriented in the transverse direction.
Regardless of whether an in-line or off-line coating technique is employed, the bond adhesion obtained between the polypropylene-containing layer and certain low melting point polymers that may serve as the coating, such as an ethylene-vinyl acetate (EVA) copolymer, may be less than adequate.
One method to improve bond strength is to corona-treat the substrate before extrusion coating the, e.g. EVA copolymer, thereon, or to ozone-treat the, e.g., EVA copolymer, before it contacts the substrate. Bond strength, however, is not always sufficient with either of these methods, and, furthermore, odor and corrosiveness are issues with ozone.
A reliable method to achieve high bond adhesion is to apply a primer layer between the substrate and the low melting point coating. Examples of suitable primers include imine-type, water-based primers and polyurethane-type primers. The application of a primer, however, makes the process (i) more complicated, because the primer solution has to be prepared and diluted, and (ii) more expensive, because the drying time for the primer limits the coating speed.
It would be advantageous to be able to provide, without the need for primer, a film structure exhibiting good bond adhesion between an extrusion-coated, low melting point polymer, such as an EVA copolymer coating, and the substrate to which the coating is applied.
In addition, it must be noted that via off-line coating, a film structure is provided wherein the low melting point coating has not at all been oriented. Via in-line extrusion coating, a film structure is provided wherein the low melting point coating has been oriented in only one direction, i.e., the transverse direction.
Attempts to manufacture a coextruded, biaxially oriented film structure comprising at least a polypropylene-containing base layer and an outer layer comprising a low melting point polymer have been hampered by the machine-direction orientation step in conventional methods of manufacturing biaxially oriented film structures. Specifically, the machine-direction orientation step may require machine-direction orientation (MDO) roll temperatures above about 220° F. (104° C.) in order to properly stretch the base layer. Therefore, a low melting point polymer may not be coextruded and machine-direction oriented by such a method because a low melting point polymer that contacts MDO rolls at such temperatures tends to disadvantageously stick to the MDO rolls. Other complications arising from contacting a low melting point polymer outer layer with relatively high temperature MDO rolls include residue build-up, optical defects, and the potential for forming holes in the film sheets that leads to breaks and production interruption.
It would therefore be advantageous to be able to coextrude and biaxially orient a film structure comprising at least a polypropylene-containing base layer and an outer layer comprising a low melting point polymer.